A physician who owns a two-provider practice asked us to show her what her billing company actually does with the money that comes in. We opened an electronic remittance from her largest commercial payer. It covered 38 claims and paid $4,112.60. She had never seen one. Within ten minutes she had asked why three claims showed "CO-45" for more than half the billed amount, why one claim paid zero with a "PR-1," and what "N130" meant. Those are the right questions, and the answers are the whole job of payment posting.
The electronic remittance advice (ERA) is the payer's formal response to a batch of claims, delivered as an X12 835 transaction, the standard file format the HIPAA transaction rules require. Every dollar the payer paid, every dollar it did not pay and the reason for each is in there, encoded in a vocabulary of group codes, claim adjustment reason codes and remittance advice remark codes. Learning how to read an ERA remittance advice is the single most useful skill a practice owner can pick up in an afternoon, because it is the only place the payer explains itself.
This is the walkthrough we give new billers and curious physicians: the structure of the 835, what the codes mean, a worked example with real numbers, and the five patterns in a remittance that mean something is wrong.
Key takeaways
- An 835 has a header with the check or EFT information, then one loop per claim, then one line per service inside each claim, with adjustments at both the claim and service level.
- The group code (CO, PR, OA, PI) tells you who is responsible for an adjustment; the claim adjustment reason code (CARC) tells you why; the remark code (RARC) adds detail.
- CO-45 is a normal contractual write-off; CO-16, CO-197 and CO-29 are denials someone has to work; PR-1, PR-2 and PR-3 are patient balances to bill.
- Billed minus allowed should equal the contractual adjustment; allowed minus paid should equal the patient responsibility plus any other adjustments, and when the math does not tie, something is wrong.
- Remittances also carry takebacks, forward balances and interest, which post differently and are the source of most unreconciled cash.
The shape of an 835
Think of the file in three layers. The top layer is the payment itself: payer name, the check or electronic funds transfer (EFT) trace number, the payment date and the total amount. Your bank deposit should match this number exactly, and if it does not, stop and find out why before posting anything. The middle layer is a loop for each claim in the payment: patient name, member ID, your claim control number, the payer's internal claim number, claim status (processed as primary, processed as secondary, denied, reversed), total billed, total paid and total patient responsibility. The bottom layer is each service line within the claim: the CPT or HCPCS code, modifiers, date of service, units, billed amount, allowed amount, paid amount and the adjustments that explain the difference.
Adjustments appear at both the claim level and the service line level, and a common beginner error is to read one and miss the other. A denial for a missing authorization often sits at the claim level and applies to every line. A bundling denial sits on the specific line that was bundled. Your practice management system flattens all of this into a posting screen, but when a posting looks wrong, the raw 835 or the human-readable remittance report the clearinghouse produces is where you go to check.
Group codes: who is responsible
Every adjustment on an ERA carries a two-letter group code that says who absorbs the amount. CO is contractual obligation: the difference between what you billed and what the contract allows, plus denials the payer says you cannot pass to the patient. PR is patient responsibility: deductible, coinsurance, copay and non-covered services the patient can be billed for. OA is other adjustment, used when neither of the first two fits, most commonly on coordination of benefits when another payer is primary. PI is payer initiated reduction, used for reductions the payer makes that are not contractual and not the patient's, such as certain penalties.
The group code drives posting. A CO adjustment is written off. A PR adjustment transfers to the patient's balance. An OA adjustment usually means a secondary claim needs to go out or a primary payer needs to be corrected. If your posting rules have a CO code transferring to patient balance, patients get statements for money they do not owe, and if they have a PR code writing off, the practice loses money it was entitled to collect. Check the rules once a year.
Reason codes and remark codes: why
The claim adjustment reason code (CARC) is a number that explains the adjustment. The list is maintained by a national committee and used by every payer, though payers apply the codes with varying consistency. The remittance advice remark code (RARC) is an alphanumeric code, usually starting with N or M, that adds detail the CARC alone does not convey. A CO-16 (claim lacks information needed for adjudication) means nothing on its own; the RARC beside it, such as N286 (missing or incorrect referring provider primary identifier), tells the biller what to fix.
| Code | Plain meaning | What to do |
|---|---|---|
| CO-45 | Charge exceeds fee schedule or contracted amount | Write off; verify against the loaded fee schedule that the allowed amount is right |
| CO-16 | Claim lacks information; see remark codes | Read the RARC, correct the claim, resubmit as a corrected claim |
| CO-29 | Time limit for filing has expired | Check for proof of timely filing; appeal or write off to a timely filing code |
| CO-97 | Payment included in another service (bundled) | Check NCCI edit and modifier; appeal with records if distinct |
| CO-197 | Precertification or authorization absent | Locate the authorization; request retro authorization if the payer allows; appeal |
| CO-B7 | Provider not certified or eligible for this service on this date | Check enrollment effective date; hold claims until loaded; appeal with enrollment confirmation |
| PR-1 | Deductible | Transfer to patient balance after confirming secondary coverage |
| PR-2 | Coinsurance | Transfer to patient balance |
| PR-3 | Copay | Apply the copay collected at the visit; bill any remainder |
| OA-23 | Impact of prior payer adjudication | Normal on secondary remits; confirm the primary payment matches |
| CO-22 | Care may be covered by another payer per coordination of benefits | Verify primary coverage with the patient; correct COB and rebill |
A worked example
Take one claim from the physician's remittance. A 99214 billed at $210 and a 36415 venipuncture billed at $25. The 835 shows the 99214 with an allowed amount of $126.40, a CO-45 adjustment of $83.60, a PR-2 of $25.28 and a payment of $101.12. The 36415 shows allowed $3.00, CO-45 of $22.00, PR-2 of $0.60 and payment of $2.40. Total paid on the claim: $103.52. Total patient responsibility: $25.88.
Now check the math. Billed $210 minus allowed $126.40 is $83.60, which matches the CO-45. Allowed $126.40 times 20 percent coinsurance is $25.28, which matches the PR-2. Allowed minus coinsurance is $101.12, which matches the payment. Everything ties. The next question is whether $126.40 is the right allowed amount, and that requires the contracted fee schedule loaded in the system. If the contract says $134.00 for a 99214, this claim is underpaid by $7.60 and the CO-45 is $7.60 too large. Nobody catches that by reading the remit alone, which is why loading fee schedules matters.
The claim that paid zero with PR-1 is simpler: the allowed amount went to the patient's deductible. The biller confirms no secondary coverage, and the balance goes to a statement. The N130 remark she asked about means the claim was processed under the terms of a specific benefit plan document; it is informational and requires no action, which is true of many remark codes and is why billers learn to sort them into "act" and "ignore."
Five patterns that mean something is wrong
First, an allowed amount that varies for the same code from the same payer across claims in the same month. That usually means the payer is applying different fee schedules to different products, or has loaded a schedule incorrectly for some providers. Second, CO-45 amounts that grow in January without a contract change, which means the payer's new-year fee schedule load differs from yours. Third, a cluster of CO-16 denials with the same remark code across many claims, which points at one registration or claim-build error, usually fixable in the practice management system once. Fourth, PR-3 copay amounts on the remit that do not match what the front desk collected, which means the eligibility copay data is stale. Fifth, claims that appear on a remittance as reversals (a negative payment followed by a reprocessed positive one, or not followed by anything), which are takebacks and need their own reconciliation.
Takebacks and forward balances deserve a word. When a payer recovers a prior overpayment, it often does so by deducting from the current remittance. The 835 shows a PLB segment (provider level balance) at the payment level with the amount and a reference to the original claim. Posting staff who do not read the PLB post all the claims as paid and then cannot reconcile the deposit to the remit.
Making remittance review a habit
You do not need to read every remit. You need three habits. Reconcile every 835 total to the bank deposit before it posts. Run a monthly report of adjustments by CARC and by payer, and look at anything that moved. And once a quarter, pull one full remittance from each major payer and read it the way we did above, checking the math on five claims against the loaded fee schedule. Practices that do this catch payer errors in the month they start. Practices that do not find them in an RCM audit two years later, often with the recovery window half gone.
For new billers, the fastest way to learn this is to post practice remittances until the codes are familiar. Our RCM training courses use real 835 structures with fictional patients for exactly that.
Questions we hear
Is the ERA the same as the EOB?
The explanation of benefits (EOB) is the paper or PDF version sent to the patient, and sometimes to the provider, in plain language. The ERA is the electronic 835 sent to the provider for automated posting. They describe the same adjudication, but the ERA has the codes and the EOB has the prose. Practices should be receiving ERAs for every payer that offers them; paper remits are slower and have to be keyed.
Why does the payer's claim number differ from ours?
Your claim control number is what your system assigned when the claim was created; the payer assigns its own internal control number (ICN or DCN) on receipt. The 835 carries both. Use the payer's number on every phone call and appeal, and keep yours for matching the remit to the original claim in your system.
The remit shows an adjustment code we have never seen. Where do we look it up?
The official CARC and RARC lists are published by the Washington Publishing Company on behalf of the X12 committee and are free to search online. Your clearinghouse portal usually includes a lookup as well. If the payer's use of a code does not fit the official description, call the payer; some apply codes in ways the list never intended.
What to do this week
- Pull one recent 835 from your largest commercial payer and reconcile the payment total to the bank deposit.
- Pick five claims on it and check that billed minus allowed equals the CO-45 and that allowed minus paid equals patient responsibility.
- Compare the allowed amounts on those five claims to your contracted fee schedule for the same codes.
- Review your posting rules for every CO, PR, OA and PI group code and confirm each routes to write-off, patient balance or a work queue correctly.
- Search the last three months of remits for PLB segments and confirm each takeback was reconciled to a specific claim.
